Spacecraft yaw control using only wheel speed measurements processed through a simple filter bank

Abstract

An attitude or orientation control system for spacecraft that provides for yaw axis error reduction and momentum dumping using a simple calculation involving only the momentum wheel speeds as input and which outputs two axis (roll and yaw) torque commands. The system utilizes the spacecraft 1) momentum wheels that store spacecraft momentum, 2) processing means for the calculation of control torques, and 3) torque actuators, e.g., magnetic torquers, that can provide a torque axis anywhere in the spacecraft roll/yaw plane, and involves three fundamental steps, 1) the input of the momentum wheel speed mearsurements, and 2) the calculation of the required torques, using the wheel speed measurements, and and 3) the outputting of the required torques in a form appropriate for controlling the available actuators.

What is claimed is: 
     
       1. An attitude control system for spacecraft having momentum wheels for spacecraft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising: means for inputting signals indicative of the momentum wheel speeds to said processor means;   means in said processing means for calculating appropriate attitude torques, using said momentum wheel speeds indicative signals, comprising: means, in accordance with a predetermined processing timing, for obtaining, from said momentum wheel speeds indicative signals, the spacecraft momentum wheel speed measurements and projecting said spacecraft momentum wheel speed measurements onto an axis in the spacecraft roll/yaw plane;   means for obtaining the spacecraft momentum on said roll/yaw axis;   a plurality of simple filters with different time constants which are selected according to the particular spacecraft and application;   means for inputting said roll/yaw plane axis momentum in said filters;   means for using the output of said filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application;   means for transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control said torque actuators; and     means, responsive to said transformed weighted linear sums, for producing output signals indicative of the calculated appropriate attitude torques in a form for controlling said torque actuators.   
     
     
       2. A system as in claim 1 wherein said roll/yaw plane axis is the Z-axis. 
     
     
       3. A system as in claim 1 wherein said filters are first order filters. 
     
     
       4. A system as in claim 1 wherein said torque actuators comprise an X-axis magnetic torquer coil and a Z-axis magnetic torquer coil, said means for inputting signals indicative of the momentum wheel speeds comprises momentum wheel tachometers, and wherein said roll/yaw plane axis is the Z-axis, said filters are first order filters, and said time constants and weighted linear sums are obtained through an optimization that uses an objective function constructed for the particular spacecraft and spacecraft application, said constructed objective function comprising the values: HZ=spacecraft Z-axis momentum due to the momentum wheels as determined solely from the momentum wheel tachometers;   HZ a  =HZ first order filtered with a long term time constant;   HZ b  =HZ first order filtered with an intermediate term time constant; and   HZ c  HZ first order filtered with a short term time constant;   and wherein,   X-axis magnetic torquer coil amps, which correspond to the value produced by one of said two weighted linear sums,=g 1  ×HZ a  +g 2  ×HZ b  +g 3  ×HZ c ,   where g 1  +g 2  +g 3  =1; and   Z-axis magnetic torquer coil amps, which correspond to the value produced by the other of said two weighted linear sums,=g 4  ×HZ a  +g 5  ×HZ b  +g 6  ×HZ c ,   where g 4  +g 5  +g 6  =1.   
     
     
       5. A system as in claim 4 wherein said long term time constant comprises multiple hours. 
     
     
       6. A system as in claim 4 wherein said intermediate term time constant comprises tens of minutes. 
     
     
       7. A system as in claim 4 wherein said short term time constant comprises minutes or fractions thereof. 
     
     
       8. A system as in claim 1 wherein said torque actuators comprise an East face magnetic torquer and an anti-earth face magnetic torquer and wherein said means for calculating appropriate attitude torques comprise means for executing the following code at 64 second intervals: HZ a  =0.997925*HZ a  +0.002075*HZ|513.520 minute time constant,   HZ b  =0.962838*HZ b  +0.037162*HZ|28.166 minute time constant,   HZ c  =0.854345*HZ c  +0.145655*HZ|6.776 minute time constant,   Xamp=8.52*HZ a  -7.97*HZ b  -0.55*HZ c     Zamp=-2.82*HZ a  -8.48*HZ b  -11.30*HZ c     where all of the variables are single precision, and   HZ=the spacecraft body Z-axis component of the momemtum due to the momentum wheels in newton-meter- seconds, +Z points toward the Earth;   Xamp=the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces Z-axis torques;   Zamp=the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces X-axis torques; and   the current is limited to a maximum of 0.4 amp for both axes.   
     
     
       9. A method for controlling the attitude of a spacecraft having momentum wheels for spacecraft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising the steps of: inputting signals indicative of the momentum wheel speeds to said processing means;   calculating appropriate torques for controlling the spacecraft attitude in said processing means using the momentum wheel speed indicative signals; and   outputting signals from said processing means indicative of the calculated appropriate attitude torques in a form for controlling said torque actuators;   and wherein said step of calculating in said processing means comprises the steps, in accordance with a predetermined processing timing, of:   obtaining, from said momentum wheel speeds indicative signals, the spacecraft momentum wheel speed measurements and projecting said spacecraft momentum wheel speed measurements onto an axis in the spacecraft roll/yaw plane;   obtaining the spacecraft momentum on said spacecraft roll/yaw plane axis;   inputting said spacecraft roll/yaw plane axis momentum in several simple filters with different time constants which are selected according to the particular spacecraft and application;   using the output of said filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application;   transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control said torque actuators; and   sending the actuator commands to said torque actuators for carrying out said controlling operation.   
     
     
       10. The method of claim 9 wherein said roll/yaw plane axis is the Z-axis. 
     
     
       11. The method of claim 9 wherein said filters are first order filters. 
     
     
       12. The method of claim 9 wherein said torque actuators comprise an X-axis magnetic torquer coil and a Z-axis magnetic torquer coil, said signals indicative of the momentum wheel speeds are input by momentum wheel tachometers, and wherein said spacecraft roll/yaw plane axis is the Z-axis, said filters are first order filters, and said time constants and weighted linear sums are obtained through an optimization that uses an objective function constructed for the particular spacecraft and spacecraft application, said constructed objective function comprising the values: HZ=spacecraft Z-axis momentum due to the momentum wheels as determined solely from the momentum wheel tachometers;   HZ a  =HZ first order filtered with a long term time constant;   HZ b  =HZ first order filtered with an intermediate term time constant;   HZ c  =HZ first order filtered with a short term time constant;   and wherein,   X-axis magnetic torquer coil amps, which correspond to the value produced by one of said two weighted linear sums,=g 1  ×HZ a  +g 2  ×HZ b  +g 3  ×HZ c ,   where g 1  +g 2  +g 3  =1; and   Z-axis magnetic torquer coil amps, which correspond to the value produced by the other of said two weighted linear sums,=g 4  ×HZ a  +g 5  ×HZ b  +g 6  ×HZ c ,   where g 4  +g 5  +g 6  =1.   
     
     
       13. The method of claim 12 wherein said long term time constant comprises multiple hours. 
     
     
       14. The method of claim 12 wherein said intermediate term time constant comprises tens of minutes. 
     
     
       15. The method of claim 12 wherein said short term time constant comprises minutes or fractions thereof. 
     
     
       16. The method of claim 9 wherein said torque actuators comprise an East face magnetic torquer and an anti-earth face magnetic torquer and wherein said calculating step comprises executing the following code at 64 second intervals: HZ a  =0.997925*HZ a  +0.002075*HZ|513.520 minute time constant,   HZ b  =0.962838*HZ b  +0.037162*HZ|28.166 minute time constant,   HZ c  =0.854345*HZ c  +0.145655*HZ|6.776 minute time constant,   Xamp=8.52*HZ a  -7.97*HZ b  -0.55*HZ c     Zamp=-2.82*HZ a  -8.48*HZ b  -11.30*HZ c     where all of the variables are single precision, and   HZ=the spacecraft body Z-axis component of the momentum due to the momentum wheels in newton-meter-seconds, +Z points toward the Earth;   Xamp=the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces Z-axis torques;   Zamp=the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces X-axis torques; and   the current is limited to a maximum of 0.4 amp for both axes.   
     
     
       17. In a satellite having an attitude control system including momentum wheels for craft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the craft roll/yaw plane, the improvement including a torques control and momentum dumping system comprising: means for measuring the momentum wheel speeds and inputting signals indicative thereof to said processing means;   means in said processing means for calculating appropriate attitude torques, using said momentum wheel speed indicative signals; and   means for producing outputs indicative of the calculated appropriate attitude torques in a form for controlling said torque actuators and dumping momentum;   and wherein said means for calculating comprises:   means, in accordance with a predetermined processing timing, for processing said momentum wheel speed indicative signals and projecting the result of said processing of said momentum wheel speed indicative signals onto an axis in the spacecraft roll/yaw plane;   means for obtaining the spacecraft momentum on said roll/yaw plane axis;   a plurality of first or second order filters with different time constants which are selected according to the particular spacecraft and application;   means for inputting said roll/yaw plane axis momentum in said first or second order filters;   means for using the output of said first or second order filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application;   means for transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control said torque actuators; and   means for sending the actuator commands to said outputs producing means for controlling said actuators.   
     
     
       18. In a satellite as in claim 17 wherein said torque actuators comprise an X-axis magnetic torquer coil and a Z-axis magnetic torquer coil, said signals indicative of the momentum wheel speeds are input by momentum wheel tachometers, and wherein said time constants and weighted linear sums are obtained through an optimization that uses an objective function constructed for the particular spacecraft and application, said constructed objective function comprising the values: HZ a  =HZ first order filtered with a 48 hour time constant;   HZ b  =HZ first order filtered with a 24 minute time constant;   HZ c  =HZ first order filtered with a 6 minute time constant;   X-axis magnetic torquer coil amps, which correspond to the value produced by one of said two weighted linear sums,=g 1  ×HZ a  +g 2  ×HZ b  +g 3  ×HZ c ,   where g 1  +g 2  +g 3  =1; and   Z-axis magnetic torquer coil amps, which correspond to the value produced by the other of said two weighted linear sums,=g 4  ×HZ a  +g 5  ×HZ b  +g 6  ×HZ c ,   where g 4  +g 5  +g 6  =1.   
     
     
       19. In a satellite as in claim 17 wherein said torque actuators comprise an East face magnetic torquer and an anti-earth face magnetic torquer and wherein said means for calculating comprises means for executing the following code at 64second intervals: HZ a  =0.997925*HZ a  +0.002075*HZ|513.520 minute time constant,   HZ b  =0.962838*HZ b  +0.037162*HZ|28.166 minute time constant,   HZ c  =0.854345*HZ c  +0.145655*HZ|6.776 minute time constant,   Xamp=8.52*HZ a  -7.97*HZ b  -0.55*HZ c     Zamp=-2.82*HZ a  -8.48*HZ b  -11.30*HZ c     where all of the variables are single precision, and   HZ=the spacecraft body Z-axis component of the momentum due to the momentum wheels in newton-meter-seconds, +Z points toward the Earth;   Xamp=the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces Z-axis torques;   Zamp=the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces X-axis torques; and   the current is limited to a maximum of 0.4 amp for both axes.